Old and New Species

THE ORIGINS OF LIFE ON EARTH

The Earth is about 4500 million years old and life began around 3500 million years ago.  We can date rocks. Fossils are found in rocks, so we can date when different organisms existed.  Fossil may be formed in various ways:

• From the hard parts of an animal that don't decay easily, eg bones, teeth, shells, claws
• From parts of organisms that have not decayed because the conditions for decay have been absent, eg fossils of animals preserved in ice
• When parts of the organisms are replaced by other materials, such as minerals, when they decay
• As preserved traces of organisms, eg footprints and burrows

Most organisms that died did not leave a fossil because the exact conditions for fossil formation weren't present.

Many early life forms had soft bodies, so they left few traces behind.  Traces which were left were likely to have been destroyed by geological activity such as earthquakes.

EXPLORING THE FOSSIL EVIDENCE

The fossil record is incomplete, but we can learn a lot from fossils that do exist.  Some organisms have changed a lot over time and others have changed very little.  Some have become extinct.  Extinction is when a species which once existed has completely died out.  Extinction can be caused by a number of factors, but always involve a change in circumstances.  

• A new disease may kill all members of a species
• The environment changes over…